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[fitsbits] which WCS keywords?
I have a question about what world coordinate system (WCS) keywords to
use for some image data that do not fit neatly into any of the three categories listed in "Representations of world coordinates in FITS," Greisen and Calabretta 2002, Astron & Astrophys, volume 395, pp 1061 - 1075 (Paper I). The three image formats for FITS files listed in Table 2 of Paper I a (a) a primary array or IMAGE extension (i.e. a conventional image), (b) a multidimensional array in a BINTABLE column, and (c) a tabulated list of pixels in a BINTABLE. Different keywords are needed for these three cases because keywords for (b) include a column number and keywords for (c) include one or two column numbers. The image format in question is for the Kepler spacecraft. An array of CCD detectors will be used to take of images of a region of the sky. See http://www.kepler.arc.nasa.gov/ Because of the large data volume, most of the data will be discarded; pixel values in a small region around each of the 170,000 or so targets will be returned from the spacecraft to Earth. The (proposed) FITS data format is a BINTABLE extension for each chip, with two columns; one column has the raw pixel values and the other column has the calibrated pixel values. It will not be possible to tell from the data file alone where a given table element is on the CCD; to avoid redundancy the pixel numbers are given in a separate reference table which is identified by a header keyword. If the pixel numbers were in the data file, this would be case (c), a tabulated list of pixels. The keywords for case (c) cannot be used for the Kepler data format, however, because the keywords include the column numbers of the pixel coordinates, and those columns don't exist. It seems to me that case (b) is reasonably close, even though the element at a given row and column is only a single value (1-D array of length 1). The value in each row really is an image, albeit a very small one! The keywords for case (b) include a column number, which in this case would be 1 for the raw data and 2 for the calibrated data. My question is whether I would be stretching the limits of the FITS standard to use the WCS keywords for a "multidimensional array in a BINTABLE column" for Kepler data. If so, is there a more reasonable alternative? One point that I haven't mentioned but may be relevant is that the values of the WCS keywords will be nearly constant over a period of three months (the spacecraft will be rolled 90 degrees every three months). There will be small, predictable shifts and scale changes due to velocity aberration, and such effects as pointing errors and temperature variations could introduce small, unpredictable shifts. Phil |
#2
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Phil Hodge wrote about Kepler data for which data and WCS information
are in separate files, if I understand correctly. What I would do is to archive the data on the ground in files with data and WCS in the same file in a BINTABLE. The files sent down from the spacecraft don't have to be FITS standard, just the archived information. -Doug Mink |
#3
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Doug,
The question was more about which set of WCS keywords to use, for the case that the data and the keywords were in the same file. My original message was probably too wordy; the question was buried by the background info! Phil |
#4
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#5
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Mark Calabretta wrote:
In splitting the files I assume that you will associate each row of one with the corresponding row of the other? Yes. Why not just have a bintable with four columns: the two pixel coords, and the raw and calibrated pixel values (and maybe other columns as well)? This is in keeping with the model established in Sect. 3.2 of Paper I. In order to save about 7 TB of archive space per year. The pixel coordinates are constant for three months, so putting them in the same file with the data would introduce a great deal of redundancy. And there will be other columns in the separate reference table, yes. The pixlist WCS keywords must be associated with the pixel coordinate values, even if they're in a separate file, not with the pixel brightness values. Specifically the "n" in keywords like TCTYPn refers to the column containing the pixel coordinate value, p_j. Right. That's why I can't use the keywords for a pixel list. You would need to add four columns to record iCRVLn for both the raw (1CRVL1, 2CRVL1) and calibrated values (1CRVL2, 2CRVL2) - the other bintable WCS keywords, being constant, could be stored in the header via the Greenbank convention. But in fact you'd be better off just storing the ra and dec in separate columns, i.e. no WCS at all. Then we would have the ra and dec for each row, but we wouldn't be able to convert from ra and dec to pixel coordinates. It would be better to add the two columns of pixel coordinates and then use the keywords for a pixel list. I was proposing to use header keywords that would define the transformation between pixel coordinates and ra & dec. The data block of the FITS file won't contain an image array, just individual pixel values, so what's missing is the connection between pixel value and pixel location on the detector. The WCS keywords will still make it possible to compute, say, the location on the detector corresponding to a given ra and dec, or vice versa. One-element arrays are within the limits of the formalism, it's just that it doen't make much sense to use them. What you have is a clear- cut case of a pixel-list. Yes, but without the pixel coordinates, and I think we've both agreed that the keywords for a pixel list cannot be used because they refer to the numbers of columns that don't exist. Phil |
#6
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